Rear brakes locking up - help

[Dsquared]

I’m trying to get my bronco back on the road after sitting in the garage for several years. I have disc/drums with proportioning valve and vacuum assist. This was an add on from one of the vendors many years ago. Ever since I did this upgrade my brakes haven’t worked that great. After much research I realized my proportioning valve was bad and when I took it of sure enough it was froze up. I bought a new one and installed it, bleed the brakes using the little tool to keep the valve centered. Brakes felt week so I’ve bleed them many times. Now the backs are locking up in the rear when I apply the brake. I backed off the shoes on the drums and that helped but they still lock up if I apply hard brake. I was wondering if my master cylinder lines were wrong going to the valve. I have the corvette master cylinder and after doing some reading here now I’m wonder if that master cylinder is for disc/disc applications and if so should I get another master cylinder or try an adjustable proportioning valve?

 

[DirtDonk]

What other modifications have been done to the Bronco?
Is it lifted? Large tires, or small? What year? Big brakes or small?

And is it a real proportioning valve/combination valve that you installed with the kit, or are you still utilizing the original H-block distribution block on the top of the frame?
Just making sure about those things. I don't believe that type of master cylinder in itself would cause the problem. Many of us use original masters on disc setups, and disc/disc masters and disc/drum Broncos, with few reports of locking rear brakes. If any...

Are your brake hoses/lines plumbed so that the rearmost reservoir (closest to the firewall) is routed to the front brakes and the front reservoir routed to the rear brakes?

Are you using the GM or Ford disc brake setup? Either way, have you checked that the calipers are able to move easily?
If Ford, the sliding surfaces might have a bit of rust on them.
If the GM setup, make sure there is still daylight showing between the caliper and knuckle where the knuckle was ground down originally. If it was not clearanced sufficiently in the beginning, after pad wear makes the caliper move you might have run out of room to move and the fronts are just jammed up. Leaving the rears to do all the work.
If you peek over the top of the front tires you should be able to see the ground through the gap between the caliper and knuckle.

How does the pedal feel? Good and solid with a short throw?

Good luck.

Paul

 

[Dsquared]

71 with original 302 and 3 speed. 3.5” lift on 35’s. It’s Gm discs up front, I’ll have to check the clearance that you asked about, the pads are pretty much new I haven t driven it much in many years. The valve is the one for disc/drums, no h block. The lines off the master cylinder are front to front and rear to rear. I was wondering about swapping them but the reservoirs on that master cylinder look the same size. Man I hope that’s it. I’ll swap them and give it a try.

 

[Dsquared]

Pedal is firm but feels like it travels to much.

 

[SteveL]

I believe the Corvette mc is a disc/disc application. You can try an adjustable proportioning valve on the rear line.

 

[gr8scott]

The lines off the master cylinder are front to front and rear to rear.

You'll be fine that way, same way I run mine. The GM guys ran them like that back in the day.

 

[jckkys]

I'm rebuilding my original combination valve because the replacements have proven leaky and and unreliable. I suspect they're Chinese. Many years ago could also be a problem. They don't last for ever and extended periods sitting idle makes them gum up faster like carburetors. I hear Wilwood makes quality adjustable combination valves.

 

[Dsquared]

Thank you for all the advice and information. I wanted to update you on what I found tonight. I did have a clearance issue with the caliper and the knuckle. That’s been like that since I put those on, explains why my brakes didn’t work so good. I corrected that this evening and the truck will stop now much better. Do you think that would affect the bleeding of the brakes? The pedal feels mushy and goes all the way to the floor. As for the rears, they are still locking up. Don’t do it every time but will when I apply hard brake. Thinking about the adjustable valve but, Im going to take a hard look at the shoes and springs and wheel cylinders and see if i missed anything.

 

[DirtDonk]

I corrected that this evening and the truck will stop now much better. Do you think that would affect the bleeding of the brakes?

Yes, definitely.
While it does not always, it's certainly capable of keeping the bleeding process from being complete because the caliper is not able to do it's thing.

The pedal feels mushy and goes all the way to the floor.

You will have to bleed it fully again, at least in the front. Now that the calipers can move, any bleeding you did before was not complete.
But you should also re-adjust the rear brakes tight again. Unfortunately, for your locking up issue.

As for the rears, they are still locking up. Don’t do it every time but will when I apply hard brake.

Unfortunately, you should re-tighten the rear brakes so that they are at least where they should be so you can continue tracking down the underlying problem. Loosening them up was just a Band-Aid to this point.
And if you have not re-bled the fronts yet and still have a mushy pedal, you should not be testing it until you get that critical step done. Any locking up of the rear is at least in part because the fronts can't do their job yet.

And with the new style proportioining valve, any time you bleed the fronts you need to pull the little needle thingy out so that fluid can flow to the fronts under even light pressure.
It's a "delay valve" to keep the fronts from being applied before the rears. It's a stability aid.

Thinking about the adjustable valve but, Im going to take a hard look at the shoes and springs and wheel cylinders and see if i missed anything.

The standard prop valve is generally pretty good, but our vehicles are no longer stock so I like adjustable ones in most cases.
With Broncos being nowhere near their original "balance" overall (bigger tires, higher center of gravity, more weight too even....) a manually adjustable prop valve is likely the best way to get the perfect balance in braking to the way the individual owner likes it.
Personally, I like the rears to lock up just a gnat's hair before the fronts. But the generally accepted industry standard is to have the fronts lock up first so you are theoretically less liable to swap ends when the skidding rear starts to come around.
I just don't like the loss of control that the fronts being locked up imparts, and like to know when it's coming. I've always found it pretty easy to keep the rear from coming around in a panic/rear-skid situation. I'm sure that's not the case in every set of conditions, but so far, so lucky for me.

Paul

 

[DirtDonk]

Oh, and while the delay valve is a great boon to stability, getting rid of it certainly makes bleeding the brakes a lot less of a hassle!

But if you hold the valve open first, bleeding should not cause any headaches.
Well, other than the one you get from trying to figure out how to hold the valve open!%)

You need to verify that yours is spring loaded outward or inward as there were originally two styles. Probably most, if not all of the aftermarket ones are "outies" though, so you need to figure out how to hold them open outward.
There used to be a tool that clipped to the tip and held it open for one-man bleeding, but I've used needle-nose Vise-Grips and other more specialized pliers to get the job done too.

Verifying which way your valve works is as easy as pulling the rubber cap off the end of the prop valve assembly and seeing which way spring pressure works. By the same token you could have someone push on the brakes and watch the valve tip move outward, or inward as the brakes are applied.
Since you don't know how much pressure is getting there though, it's best to verify manually I think.

Paul

 

[Dsquared]

I have the alignment tool that threads into the center of the valve. I think it holds the valve in position so it cannot shift forward or backward during the bleeding process. That’s what I have been using. I put my switch back on last night and the light has not come on, I’m guessing the valve is still centered but I could test it when I bleed the fronts again. When that valve moves under pressure is it easy to push back to center?

 

[DirtDonk]

Nope, different valve and different tool.
That centering tool thingy is great, but it's just for the shuttle piston to keep the dash lamp from coming on.
Also, in theory at least, the shuttle piston does not block or impede flow to the effected circuit, so has no bearing on bleeding.

The one I'm talking about is sticking out of one of the ends (usually pointed front, because it's in the front circuit only) under a rubber boot. No electrical wires...
This is a spring-loaded valve that closes off fluid flow to the front brakes (completely) under low pedal pressure. While the rear brakes are acting under even slight pressure, once a threshold is reached this valve opens up and allows full flow to the front brakes.
Completely different circuit.

Paul

 

[Dsquared]

I looked at a cutaway diagram, i understand what your describing. I’ll take a look and report back after I’ve caught up on the resent suggestions. Thank you again for all the info. Very helpful.